The present invention relates to a printed circuit board testing device and more particularly to a device for testing printed circuit boards comprising standard repetitive patterns. The invention especially applies to testing printed circuit boards comprising metallized regions adapted to receive integrated circuits connected according to the so-called surface mounting technology (SMT).
Over the last years, for mounting components on printed circuit boards (PCB), the SMT technique is being increasingly used while the insertion mounting technique becomes obsolete. In the insertion technique, each component pin passes through a hole provided in the PCB and wave soldering is achieved on the rear side of the PCB. In surface mounting technique, the component pins are folded so that their extremities are parallel to the mounting plane or tangent to the latter along at least a portion of their length. These folded extremities are laid on metallized regions provided on a PCB and are soldered on the surface, for example by wave soldering.
Among the many known advantages of the SMT, it can be noted that the component pins and the metallized regions on which they are laid can be miniaturized with respect to the case of insertion mounted components.
However, new difficulties regarding the testing of these PCBs before component mounting result from this advantage.
In fact, before mounting components on a PCB, the manufacturer first desires to check for errors in the manufacturing of PCBs. It is essential that this testing of the bare PCB is carefully carried out because, if there is an initial defect in the PCB, there is a risk of wasting all the components that will be subsequently mounted thereon or at least to cause relatively hazardous and expensive disassembly operations.
In order to properly test a PCB, it is necessary that all the metallized regions intended for connecting the components be accessible for equipotential testing.
To couple a testing device and the testing circuits that it incorporates with the metallized regions of a PCB, various means are known, among which the most widely used up to now is a so called "nail bed" connection system. In this system, metallic needles, also called nails, or pins, are pressed against the printed circuit locations to be tested and are provided with a spring system. The needles are generally guided by one or several guiding perforated plates to allow an exact contacting of the locations on the PCB to be tested. The lower guiding plate can be perforated by a digital control system controlled by data generated, for example, by a magnetic tape, corresponding to the data that allowed to determine the positions of the metallized regions on the PCB. Thus, the use of a nail bed implies that the user is equipped with various devices including particularly the nail bed itself, and mechanical and computerized means for perforating the lower guiding plate.
When it is desired to test the PCBs provided for surface mounted components (SMCs), the distance between the pins of the components becomes too small for contacting each contact region directly with one of the needles.
Thus, it is necessary to add on a PCB various test points coupled by connections to each contact region in order to render the spacing compatible with a nail bed.
Such a solution is pernicious because, on the one hand, it complicates the PCB pattern and, on the other hand, it does not allow the testing of the defects existing between the added test points and the contact regions themselves, whereas it is generally the closest- spaced regions of the PCB and therefore the regions that should be tested with greatest care.